Elsevier

Journal of Supramolecular Chemistry

Volume 1, Issue 1, January–February 2001, Pages 7-16
Journal of Supramolecular Chemistry

Modulation of the electronic communication between two equivalent ferrocene centers by proton transfer, solvent effects and structural modifications

https://doi.org/10.1016/S1472-7862(01)00007-7Get rights and content

Abstract

A series of six new dinuclear ferrocene compounds with the bridging fragment CH2–N(R)–CH2 were prepared and characterized. Bis(ferrocenylmethyl)-t-butylamine (1), bis(ferrocenylmethyl)-isopropylamine (2), bis(ferrocenylmethyl)-hexylamine (3), bis(ferrocenylmethyl)-p-methoxyaniline (4), bis(ferrocenylmethyl)aniline (5) and bis(ferrocenylmethyl)-p-nitroaniline (6), as well as the quaternized amine bis(ferrocenylmethyl)hexylmethylammonium hexaflurophosphate (3+·PF6), were investigated using electrochemical techniques, 1H NMR spectroscopy and single crystal X-ray diffraction analysis. The voltammetric data indicate that the extent of electronic communication between the equivalent ferrocene centers increases in low polarity solvents and when bulky aliphatic groups are covalently attached to the central tertiary nitrogen. The observed experimental data are consistent with through-space communication effects, essentially electrostatic in nature. The extent of electronic communication can be modulated by reversible and irreversible reactions in the solution phase. Protonation or methylation of the tertiary nitrogen in the middle of the bridge disrupts the electronic communication between the ferrocene residues.

Introduction

Electronic communication effects between equivalent redox centers have been the subject of extensive research work and led to numerous reports in the chemical literature.1 Due to their synthetic accessibility, multinuclear metallocene compounds have played an important role in this area of research.2 Recently, we reported a new type of dinuclear ferrocene compound containing a CH2–N(R)–CH2 tether.3 These compounds display a moderate degree of electronic communication between their two identical ferrocene residues, which essentially disappears upon protonation or methylation of the central tertiary nitrogen in the tether.3 In order to better understand this finding, we have prepared a series of three compounds having aliphatic R groups with variable degrees of steric bulk (compounds 1, Chart 1, Scheme 1 and 3; see Chart 1 for structures). We have also prepared another three compounds (4, 5, and 6; Chart 1) having aromatic R groups to investigate the influence of electron withdrawing/releasing effects on the communication between the ferrocene centers.

In spite of the abundance of dinuclear ferrocene compounds that are already described in the literature,2 the compounds reported here are attractive due to the following reasons: (i) their ease of preparation, (ii) the presence of a centrally located nitrogen atom in the tether connecting the two ferrocene groups, and (iii) the possibility to manipulate (reversibly or irreversibly) the extent of electronic communication between the ferrocene centers using the reactivity of the tertiary amine functional group. In this paper, we provide a detailed account of the synthesis and electrochemical properties of these compounds, as well as relevant 1H NMR spectroscopic and single crystal X-ray diffraction data.

Section snippets

Synthesis

The preparation of the dinuclear ferrocene compounds Chart 1, Scheme 1 is based on the known dissociation in polar solvents of the (ferrocenylmethyl)trimethylammonium ion to produce trimethylamine and the corresponding ferrocenylmethylene carbocation.4 In the presence of nucleophiles, this carbocation gives rise to ferrocenylmethylated products.4, 5 Therefore, reaction with a primary amine easily yields the corresponding bis(ferrocenylmethyl) tertiary amine (see Scheme 1) plus residual amounts

Materials

All solvents and chemicals for synthesis were commercially available and were used without any further purification. Solvents were removed on a rotary evaporator connected to a water-aspirator and the remaining traces were evaporated on a vacuum oven typically set at 70 °C overnight. For electrochemistry, acetonitrile and dichloromethane (99.9%, HPLC grade) were purchased from Aldrich.

Methods and instrumentation

1H NMR spectra were recorded on a Varian VXR-400 spectrometer and chemical shifts were measured with reference

Conclusions

In the solution phase the dinuclear ferrocene compounds 1 and Chart 1, Scheme 1 display different degrees of electronic communication that correlate well with geometric parameters describing the average distance between their ferrocene centers in the solid state. Although we could not determine the crystal structure of 3, the unbranched nature of its hexyl substituent fits very well with the trend that extrapolates from the relative steric bulk of the substituents in compounds 1 (t-butyl) and

Supporting Information

Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as CCDC 155780–155784 for compounds 1, Chart 1, Scheme 1, 3+ 6 and 5, respectively. Tables of atomic coordinates, bond lengths and angles, and anisotropic displacement parameters for these five compounds (Tables S1–S15).

Acknowledgements

The authors are grateful to the NSF (to AEK, CHE-9982104) for the generous support of this research work. T.R. acknowledges the CCD diffractometer fund from the University of Miami. The authors also wish to thank Ms. Eden Pacsial for assistance in the collection of the X-ray data set for 3+.

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